Method of producing porous paper



Patented Apr. 21, 1953 UNITED STATES PATIZIV'I OFFICE METHOD- OF PRODUCING POROUS PAPER Daniel SL- Diefienderfer, Easton, Pa., and Andrew Loy Moore Bixler, Milford, N. J.,. assignors to Riegel Paper Corporation, New York, N. Y., a corporation of New Jersey No Drawing. Original application February 4,1 1947, Serial No. 726,446. Divided and this ap plication April 14, 1949, Serial No.- 87,564

3 Claims. (01. 92*21) method. of producing. porous paper.

More particularly the inventionrelates: to a method of producing a strong, highly porous paper having. special. properties which. make it valuable fOr use for filtering air as in vacuum cleaner bags.

According to the present invention, admixed leather fibers and. cellulose fibers-in suitable proportions are formed into a wet sheet and the wet sheet? is subjected to a heating treatment while still" wet to effect shrinkage, decomposition and change in the leather fibers: toproduce a. special porous sheet of paper containing the cellulose fibers bound together by the modified leather particles.

Particularly advantageous porous paper canibe made from admixtures of leather fibers and long cellulose fibers inapproximately equal propertions, but these proportions can be somewhat varied,.for example, between 40 and 60 leather fibers and between 60 and 40% of long cellulose fibers for the production of highly porous paper adapted for filtering air. A somewhat lower pro portion of leather fibers, down to' around 20%, and a somewhat higher proportion, up? to around 70%, can be used to give paper of modified properties which is less valuable for: air filtering purposes but adaptedfor other purposes:

The leather fibers used in making. the new paper may be ordinary leather fibersrobtained by shredding or fiberingleather waste'or. scrap which.

isshredded or divided so that the fibersare' sepa rated more or less completely and present a fibrous structure and appearance adapting them The cellulose fibers may be of'any-type that can be formed'in'to For the" purpose of for use in. paper manufacture;

a web on a papermachine. the product herein. described, long, strongcellulose 'fibers are bestadaptedl Since. the leather" fibers are dark in color. and darkeni even moreduring shrinkage and decomposition,sunbleached,

fibersare-well adapted. Among such types'which arefound very suitable are strong kraft's; rope and jute;

The admixture of leather and cellulose fib'ers' and the production of anadmixed pulp therefromcanbe carried out in ordinary pulp making equipment. Thus the pulp may be prepared by the following procedure.

The cellulose fibers such as unbleached Kraft selected. for. a particular applicationdepends on the porosity and strength properties desired in the final sheet. In this particular case, we have found a freeness below 500 cc. to be undesirable. The leather fibers are next added to the beater, using e. g., tanned scrap leather in a fibrousform. Different percentages of leather. based on thetotal dry weight, will. givevarying amounts of porosity anddryand wet strength in the final sheet. For the production of vacuum cleaner bag paper a mixture of approximately 50% leather fibers andt50% cellulose fibershas been found to give the desired high porosity and sufiicient overall strength. After the addition of the leather fibers to the beater, the mixtureisrun in the beater only long enough to thoroughly wet the leather fibers,.todistribute them evenly through the furnish and. reduce the stock to a slowness that will give the desired propertiesinthe final sheet.- The resulting freenessofthe furnish is intherange'of about500 cc. to200 cc; depending on the freeness of the material first furnished inthe beater and the length of time the combined furnish is: run. The stock is now ready for the paper machine.

The pH of the stock should not be changed to any great extent from the normal resulting pH produced by the leather fibers, other fibers and water in. the beater. This normal is in the range of around 4.5 in the beater to 6.5 in the headbox. A lowering of thepH below about 4.0

tends to retard the shrinkage and decomposition.

of the leather during the subsequentheatingcon. the steam dryers with resulting. lowering in porosity; while too high a pH. around 8.0 or

higher, tends to causethe: leather to decompose or dissolve in the stock mixer, thus tending to lose the eiiect of theshrinkage and decomposition which is desired onthe steam. dryers.

The prepared furnish from the beater is supiplied to an ordinary: paper: making; machine and" a wetsheet of paper is produced; The: drying of this: sheet in the ordinary wayon" a paper machine does not produce" the new porous paper.

To produce -thedesired porous paper it is important" to. subject the wet sheet to a: high1temperature while the leather fibers are still wet.

It appearsthat the wetleather fibers; on being heatedtea-*sufiiciently' high-temperature tend tosoften" andswell and to decompose, becoming somewhat gelatinous so that they; adhere to the adjacent cellulose fibers. In order: to produce the new porous. paper extreme shrinkage ofathe leather fibers and more or less decomposition'on' 160 F., when a marked shrinkage and more or less decomposition of the leather fibers takes place. This shrinkage is of such a degree that irregular voids are created in the sheet, not only by the loss of volume of the leather fibers, but by the effect of the leather in distorting the cellulose fibers to which it is adhered. The leather remains gummy and sticky during the shrinking and appears to remain tacky and gummy until it becomes quite dry.

As opposed to ordinary paper making practice, in which the first dryers are running at a lower temperature with gradual increase up to a higher temperature in the last dryers, the heating of the wet sheet containing the leather fibers is effected by using a high temperature on the first dryers so that the necessary heating to a high temperature and swelling and decomposition will take place before the sheet is dried. The temperature of the first dryers in the drying system over which the wet composite paper sheet is passed should have a surface temperature of around 250 F. or higher.

When such a high temperature dryer is used the composite wet sheet is rapidly heated to a temperature, e. g., around 160 F. or somewhat higher so that prior to the actual drying and removal of water the leather fibers are caused to soften and swell and become gelatinous and adherent to the cellulose fibers. The fiber structure of the leather is largely though not completely lost during this heating and drying operation. Some of the leather material apparently forms an adhesive solution and there is probabh some complete thermal decomposition. The leather remaining is distributed through the sheet as a mixture of hard, amorphous particles and fibers; but because of the cellulose fibers the sheet as a whole retains sufiicient flexibility and strength. The dried sheet is very porous and has a rough surface due to the changes that take place in the leather during the high temperature heating and drying.

One of the important properties imparted to the sheet by the gelatinized, shrunk and dried leather is that the leather cements the cellulose fibers together with a bond that retains a considerable proportion of its dry strength when the resulting paper is wet. This imparts to the sheet What is commonly called in papermakers language wet strength. In ordinary paper made of cellulose fibers the bond strength between cellulose fibers is reduced to practically nothing when a sheet becomes wet and a sheet is said to lack wet strength. But the action of the modified leather fibers; according to the present invention, gives a paper having wet strength which makes the paper desirable for use under conditions where it may become wet during use. The improved highly porous sheet of paper produced by the present invention combines this high porosity with a wet strength comparable to or superior to that obtained with the ordinary wet strength agents used in the paper making industry.

The porosity of the paper sheet resulting from carrying out the invention has characteristic and distinctive properties. The shrinking of the swollen, gelatinized leather fibers on heating to effect partial decomposition causes shrinkage to such a degree that irregular voids are created in the sheet. These voids are created not only by the loss in volume of the leather fibers but by distortion of the cellulose fibers caused by the gummy and sticky leather fibers as they shrink and partially decompose. The resulting dried sheet has a characteristic rough surface due to the changes in the sheet which take place in the leather fibers as they swell and shrink while adhering to the cellulose fibers. The sheet surface becomes hard and dry during the heating and drying operation, with an irregular broken surface. The sheet is not only very porous but microscopic examination indicates that the pores in the sheet are of varying size.

The present invention is particularly valuable for producing paper for use in filtering air, for example, for use in making vacuum cleaner bags, where it is important to filter dust and dirt from the air and permit the air to pass through. The variation in pore size in the sheet, together with the rough surface, greatly retards the increase in resistance to air fiow as solid material (dust) is filtered out of the air on the surface, preventing the dust from collecting in a continuous, impervious film.

In contrast with porous sheets of paper made entirely of cellulose fibers with low initial resistance to air flow, where the resistance increases rapidly as solid particles collect on the terferes with its filtering action, the improved porous paper made in accordance with the present invention, with its rough, uneven surface and is characteristic interior porosity, permits effective filtering of solid material (dust) for prolonged periods of time without the formation of a continuous, impervious film.

The new paper sheet produced by the present method can be somewhat further improved in wet strength by treating the sheet after it is formed with added wet strength agents. Such agents should not in general be added at the beater or headbox since the acid condition required in such cases interferes with the conversion of the leather on subsequent heating and tends to reduce porosity. But by applying an added wet strength agent by a size press applica-- tion after the composite sheet has been formed and dried, somewhat increased wet strength can be imparted to the sheet without the loss of the desirable high porosity of the sheet.

The important characteristics of composite paper made in accordance with the new method will be illustrated by tests carried out on sheets made with varying proportions of leather and cellulose in the furnish which has been formed into a sheet and heated to bring about the shrinkage and decomposition of the leather fibers by heating the wet sheet to an elevated temperature at an early stage of the drying operation.

Sheets of paper were made from leather fibers and kraft fibers in the proportions indicated in the following table and tests were made for porosity, the time shown in the table being for porosity for four sheets, 400 cc. drop as measured with the Gurley Densometer. The sheet weight was 50/500 from a furnish of 300 cc. freeness. The following results were obtained:

aeeacsa d A further series of tests were carried out to show the effect of f'reeness'onporosity of a sheet made: of 50% leather and.50;% kraft fibers, with the. following. results:

Porosity 4 Freeness' sheets/400cc.

Sec. Ilessthanul.

The improved wet strength imparted to the sheet is illustrated by the following table, the sheets being those above described made from varying percentages of leather fibers and kraft fibers and the wet strength being indicated as retention of Mullen strength by the wet sheet with the varying percentages of leather fibers in the furnish:

Percent Furnish Retention of Mullen leather, 100% kraft 0 10% leather, 90% kraft 20% leather, 80% kraft.-. ll 30% leather, 70% kraft. 20 40% leather, (0% kraft- 25 50% leather, 50% kraft. 30 60% leather, 40% kraft 35 70% leather, 30% kraft 50 The further improvement resulting from the treatment of the finished sheet by the addition of wet strength agents, applied in the size press to a sheet made of 50% leather and 50% kraft fibers with a furnish of 350 cc. freeness is indicated by the following table:

Percent Wet Strength Agent Retention of Mullen 51"? a2 miitraaataatage-(static? 67 From these tests it will be evident that a very marked increase in porosity is obtained even with to 30% leather fibers admixed with the kraft fibers, while a particularly porous sheet is obtained with around equal proportions of leather and kraft fibers. It will also be seen that the new method produces a sheet of paper which retains a marked wet strength which can be somewhat further improved by further treatment of the finished sheet with other wet strength agents.

It will thus be seen that the present invention provides an improved sheet of paper characterized by high and distinctive porosity, having a hard, rough surface and containing long cellulose fibers bonded together and spaced apart by the modified leather fibers which are caused to pass through a swelling and gelatinizing and decomposing and shrinking treatment to give irregular shrunk particles and fibers of leather in the final product which bind the cellulose fibers together and also hold them spaced apart to give a characteristic, irregular porosity to the sheet.

It will further be seen that the composite sheet made in accordance with the present method, particularly when about equal proportions of leather and cellulose fibers are used, forms a 6' valuable filter paper for filtering air and for use in vacuum cleaner bags toreplace the ordinary cloth bags, commonly usedfor thatpurpose.

It will also be seen that the invention provides an improved porous sheet which combines high porosity with suificient strength and that the sheet retains itsstrength to avery large extent when wet, thusbeing adapted to use under conditions wherethe papermay become wet in use. And it will further be-seenthat still further improved wet strength can be-imparted by the subsequent treatment of the paper, after the completion of its formation, by other" wet strength agents.

The present invention provides an improved method which can be carried out in ordinary paper making apparatus and by ordinary paper making procedures, except for the important change in procedure involved in heating the wet sheet to a high temperature to effect swelling and gelatinizing and shrinkage and partial decomposition of the leather fibers. This important modification of the present paper making operation is however readily carried out by using high temperature in the first drying cylinders on which the wet paper is passed, with heating of the cylinders to temperatures around 250 F. or higher and with heating of the interior of the sheet to temperatures around F. or somewhat higher. This rapid and immediate heating of the wet sheet prevents the drying of the leather fibers at low temperature and insures that the leather fibers are subjected to the necessary high temperature while still wet to cause them to adhere to the cellulose fibers and to shrink into irregularly shaped particles which serve as binding and spacing particles for binding the cellulose fibers together while holding them in spaced condition to give a characteristic and distinctive and desirable interior porosity and a characteristic porous and rough surface.

This application is a division of our application Serial No. 726,446, filed February 4, 1947, now abandoned.

We claim:

1. The process of producing paper of high porosity and wet strength which consists of admixing leather fibres and long cellulose fibers in proportions such that the leather fibers make up from about 20% to 70% of the mixture, forming a wet sheet from a pulp of such admixed fibers, rapidly heating the resulting wet sheet and the leather fibers contained therein to a high temperature of at least around 160 F. but insuificient to char the fibers while said leather fibers are still wet, to cause adhesion of the leather fibers to the cellulose fibers and shrinking and partial decomposition and gelatmizmg of the leather fibers, said rapid heating of the wet sheet being effected by passing the sheet first over drying rolls heated to a surface temperature of at least about 250 C. and subsequently drying the resulting sheet.

2. The process according to claim 1, in which the leather fibers comprise from about 40% to 60% of the mixture.

3. The process according to claim 1 in which approximately equal proportions of leather fibers and long cellulose fibers are used.

DANIEL S. DIEFFENDERFER. ANDREW LOY NIOORE BIXLER.

(References on following page) References Cited in the file of this patent UNITED STATES PATENTS Number Name Date Clapp May 10, 1932 Schwartz Aug. 20, 1935 Katze-Miller Oct. 8, 1935 Ellis Apr. 7, 1936 Ives Jan. 11, 1938 Mason Dec. 13, 1938 Mason Oct. 29, 1940 Lynam Apr. 1, 1941 FOREIGN PATENTS Country Date Great Britain of 1856 Great Britain of 1856 Great Britain of 1870 Great Britain June 28, 1940 OTHER REFERENCES Manufacture of Pulp & Paper, 3d. ed., vol. V., sec. 1, pp. 199, 203 (1939), pub. by McGraw-Hill,

New York. 

1. THE PROCESS OF PRODUCING PAPER OF HIGH POROSITY AND WET STRENGTH WHICH CONSISTS OF ADMIXING LEATHER FIBRES AND LONG CELLULOSE FIBERS IN PROPORTIONS SUCH THAT THE LEATHER FIBERS MAKE UP FROM ABOUT 20% TO 70% OF THE MIXTURE, FORMING A WET SHEET FROM A PULP OF SUCH ADMIXED FIBERS, RAPIDLY HEATING THE RESULTING WET SHEET AND THE LEATHER FIBERS CONTAINED THEREIN TO A HIGH TEMPERATURE OF AT LEAST AROUND 160* F. BUT INSUFFICIENT TO CHAR THE FIBERS WHILE SAID LEATHER FIBERS ARE STILL WET, TO CAUSE ADHESION OF THE LEATHER FIBERS TO THE CELLULOSE FIBERS AND SHRINKING AND PARTIAL DECOMPOSITION AND GELATINIZING OF THE LEATHER FIBERS, SAID RAPID HEATING OF THE WET SHEET BEING EFFECTED BY PASSING THE SHEET FIRST OVER DRYING ROLLS HEATED TO A SURFACE TEMPERATURE OF AT LEAST ABOUT 250* C. AND SUBSEQUENTLY DRYING THE RESULTING SHEET. 